The invention relates to the field of treads for tires, and in particular to treads, the elements in relief (blocks or ribs) of which are provided with incisions.
It is known to provide the tread of a tire, whether the latter be intended to be fitted on a passenger vehicle or on a heavy vehicle, with a tread pattern in order to obtain satisfactory adhesion performance, particularly on ground which has been made slippery by the presence of a liquid. A tread pattern comprises blocks (sections of rubber defined by grooves) and/or ribs (sections of rubber defined by two grooves extending substantially in the same direction) defined by grooves formed by molding into the thickness of the tread.
In order to improve the adhesion performance of a tread, it is known to use a large number of incisions made on the tread pattern elements either during the molding of said tread pattern or after molding and vulcanization of said tread.
xe2x80x9cGroovexe2x80x9d is understood to mean a cutout of an average width of at least 2 mm, and xe2x80x9cincisionxe2x80x9d a cutout of lesser width, that is to say, of an average width of less than 2 mm. Depending on the desired effect, these cutouts are of greater or lesser depth relative to the thickness of the tread.
It is also known that although incisions made on an element in relief (rib or block) which open onto the edges of said element in reliefxe2x80x94that is to say, which open into the grooves defining said elementxe2x80x94promote flow of fluid towards these grooves, they result in a substantial reduction in the rigidity of the regions of the element in relief close to the edges of this element. The rigidity discussed here corresponds to the rigidity of deformation of the tread pattern element subjected to tangential forces exerted on its surface of contact with the ground.
In order to overcome this disadvantage, it was proposed to produce incisions which do not open onto the edges of the tread pattern elements (such incisions may be referred to as xe2x80x9cblindxe2x80x9d incisions).
Unfortunately, this arrangement results in an increase in the traveling noise of the tire equipped with a tread provided with such a tread pattern, given the presence of a volume of air trapped in the blind incisions, this increase being all the greater, the larger the number of incisions.
Japanese Patent Application published under No. 62-268709 proposes manufacturing a tire which when new comprises at least one incision of very low depth which crosses the trace of the blind incisions. Although this arrangement is advantageous when the tire is new, it disappears very rapidly as the tread becomes worn.
One object of the present invention is to propose a tread pattern which has excellent adhesion performance on slippery ground, that is to say, which is provided with a large number of ridges, and which does not have the disadvantages which have been set forth above.
In order to achieve the object set forth above, the invention proposes a tire tread, said tread being provided with a tread pattern formed by elements in relief defined by grooves. Furthermore, a plurality of the elements in relief are provided with blind incisions, that is to say, incisions which do not open onto the edges of said elements in the grooves.
The tread pattern element according to the invention furthermore comprises at least one cutout which opens onto at least one edge of the tread pattern element and which has a trace of average orientation such that, on the running surface, each blind incision crosses at least one open-ended cutout of the same element, whatever the level of wear of the tread. The tread pattern according to the invention is characterized in that each of the open-ended cutouts has a depth substantially equal to the depth of the blind incisions which it crosses and is filled with a filler material, said filler material having the property of mechanically connecting, at least when new, the walls of the filled cutout and wearing more rapidly than the material constituting the tread. Under the action of frictional contact with the ground during travel, the filler material is gradually eliminated to form a channel of a depth which is very low but, however, sufficient to enable part of the air trapped in the blind incisions to escape.
xe2x80x9cChannel of low depthxe2x80x9d is understood to mean a channel, the depth of which is of the order of magnitude of the width of the open-ended incision in question. xe2x80x9cCutout of depth substantially equal to the depth of the blind incisionsxe2x80x9d is to be understood to mean that this cutout has a depth equal to at least 80% of the depth of said blind incisions; it is also possible for the depth of the filled cutouts to be greater than the depth of the blind incisions which they cross.
Preferably, the width of the open-ended cutouts is at least equal to 1.5 times the width of the blind incisions which they cross, so as to have a satisfactory effect of evacuating the trapped air.
It is understood that, in order not to modify and lower the rigidity of the elements in relief of the tread pattern according to the invention, the open-ended cutouts are filled with a material which ensures a suitable mechanical connection of the opposing rubber walls defining said cutouts.
The tread pattern according to the invention has the advantage of having a large number of ridges formed by the grooves and blind incisions and also by the open-ended cutouts. The crossing of the blind incisions and of the open-ended incisions on one and the same tread pattern element, combined with a greater rate of wear for the material for filling the open-ended cutouts than for the tread pattern element has the effect of forming a channel for evacuating at least part of the air trapped in the blind incisions at the time of passing into the contact imprint of the tire with the ground. This is possible without the rigidity of the tread pattern being adversely affected thereby, and consequently without the wear performance being reduced.
Furthermore, since the open-ended cutouts have depths at least equal to 80% of the depth of the blind incisions, the mechanism described continues regularly during the entire useful life of the tire (and in particular until the wear limit of the tread is reached) and thus ensures the lasting desired traveling noise performance.
xe2x80x9cFiller materialxe2x80x9d is to be understood to mean a material:
which provides a satisfactory mechanical connection between the opposing walls of the cutout in which it is present (at least initially in the use of the tire),
and which has a rate of frictional wear during travel which is greater than the rate of wear of the rubber mix forming the tread.
More generically, it is also advantageous to apply the teaching of the invention to a tread comprising elements in relief having at least one incision which opens only onto one edge of said element, for it has been noted that, under certain traveling conditions, the evacuation of the air included in this type of incision is not sufficient, and that this results in a significant increase in the noise level.
The characteristics and advantages of the invention will be better understood with reference to the variant embodiments described hereafter, accompanied by the drawings.